1. Field of the Invention
The invention relates to an automatic focus adjusting apparatus suitable for use in a video camera or the like.
2. Related Background Art
In recent years, in an image pickup apparatus such as a video camera or the like, an automatic focus detecting apparatus (hereinafter, abbreviated to AF) is indispensable. However, as such a kind of AF, an attention is paid to an automatic focus detecting system for extracting a signal according to a sharpness of an object from an image pickup signal and for evaluating and for executing a focus detecting operation of an optical system.
There are various types as such a kind of AF system. A fundamental circuit construction of an AF apparatus to which the invention can be applied will be first described with reference to FIG. 1. An object image is formed by a focusing lens 1 and is formed onto the image pickup surface of an image pickup element 4 such as a CCD or the like through a zoom lens 2 and an iris 3. The object image is photoelectrically converted into a video signal by the image pickup element 4. The image pickup element 4 is periodically driven in the direction of an optical axis by an image pickup element driving circuit 28 which is driven at the timing of a reference clock generated from a timing generation circuit 7, thereby finely changing (wobbling) an image forming state. The video signal converted by the image pickup element 4 is amplified by a preamplifier 5 and is subjected to predetermined processes such as gamma correction, blanking process, addition of a sync signal, and the like by a process circuit 6 and is converted into a standardized standard video signal and is also supplied to a band pass filter (hereinafter, abbreviated to BPF) 8. The BPF 8 extracts the high frequency component which changes in accordance with the focusing state in the video signal, namely, sharpness. Subsequently, only the signal corresponding to a focal point detecting region set at a predetermined position in the screen is allowed to pass by a gate 9. The signal is rectified and the peak value is held by a peak detection circuit 10, so that a signal S.sub.0 is generated. A synchronism detection circuit 11 synchronism detects the signal S.sub.0 at a timing of the reference clock generated from the timing generation circuit 7 and generates a signal S.sub.1 indicative of the focusing state responsive to a micro wobbling in the optical axis direction of the image pickup element 4. The output S.sub.0 of the peak detection circuit 10 is delayed by 1 V (V denotes a vertical sync period) by a 1 V delay circuit 12. The delayed signal is compared with the non-delayed signal S.sub.0 by a comparing circuit 13, so that a signal S.sub.2 to judge whether the signal is at present climbing up or down a mountain is obtained. Further, the signal S.sub.0 is also supplied to a comparator 14 for mode decision. When the value of the signal S.sub.0 is larger than a threshold value V.sub.H, a switch 15 is switched to the signal S.sub.1 side. The signal S.sub.1 responsive to the wobbling is applied to a motor driver 16, a motor 17 is driven, and the focusing lens 1 is moved, thereby performing the automatic focusing operation in the closed loop mode. On the contrary, when the value of the signal S.sub.0 is equal to or smaller than V.sub.H, the switch 15 is switched to the mountain climbing signal S.sub.2 side and the signal is supplied to the motor driver 16. Thus, the automatic focusing operation is executed in the mountain climbing mode using no wobbling operation.
The automatic focus adjusting operation will now be further described with reference to FIGS. 2A to 2C. FIG. 2A shows the output level S.sub.0 of the peak detection circuit 10 to the position of the focusing lens 1 in the in-focused/unfocused state. When the focusing lens is located on the near distance side than the in-focused point, a response signal A.sub.N to the wobbling of the image pickup element is detected. On the contrary, when the focusing lens is located on the far distance side than the in-focused point, a response signal A.sub.F to the wobbling of the image pickup element is detected. A response signal A.sub.M is detected in the in-focused state. The phase of signal when the lens is located on the near distance side is opposite to the phase of signal when the lens is contrarily located on the far distance side, front focus/rear focus information can be obtained. By sync detecting those signals at a reference timing of the output of the timing generation circuit 5, the signal S.sub.1 shown in FIG. 2B is derived. That is, since the phase of the signal A.sub.N which is derived when the focusing lens is located on the near distance side is the same as the phase of the reference timing, a positive signal is generated as a sync detection output S.sub.1. Since the phase of the signal A.sub.F which is obtained when the focusing lens is located on the far distance side is opposite to the phase of the reference timing, a negative signal is generated as a sync detection output S.sub.1. On the other hand, the signal S.sub.1 is at almost 0 level in the in-focused state. Therefore, by supplying the signal S.sub.1 to the motor driving circuit 16, the motor 17 can be driven. The input signal to the motor driver in the closed loop has been described above. However, the signal S.sub.1 has drawbacks. When the object image is fairly unsharp, the signal S.sub.0 in FIG. 2A decreases and the output S.sub.1 in FIG. 2B also decreases, so that the motor speed is reduced. Therefore, when the value of signal S.sub.0 is equal to or smaller than V.sub.H, the signal S.sub.1 is not used but the comparison signal S.sub.2 which has been compared with the 1 V preceding signal S.sub.0 as shown in FIG. 2C is used. Since the signal S.sub.2 is the comparison output, it is the signal having a predetermined value and only a code to drive the lens. The above operating mode is the mountain climbing mode. When the positive signal is supplied, the motor driving circuit 16 drives the motor at a speed proportional to the absolute levels of the signals S.sub.1 and S.sub.2 in the far distance direction from the near distance side. When the negative signal is supplied, the motor is likewise driven in the near distance direction from the far distance side. Due to this, the focusing lens is moved. Therefore, when the object image is fairly unsharp, the focusing lens is moved in the in-focusing direction at a high speed of S.sub.2. When the focusing lens approaches the in-focused point, the operation to focus the object image to the in-focused point at which the signal S.sub.1 is equal to 0 is performed at a speed of S.sub.1, thereby executing the automatic focus adjusting operation.
In the conventional example, however, the switching between the closed loop mode using the response of the modulation signal and the mountain climbing mode which doesn't use the response of the modulation signal is executed in dependence on whether the output level of the band pass filter exceeds a predetermined value or not. Therefore, when a slope of mountain to an in-focused position of the signal indicative of an in-focused degree changes due to the position of the zoom lens or an iris value (that is, when a depth of field changes), for instance, when the focal distance is small and an angle of view is wide, the depth of field becomes deep. Therefore, a shape of mountain of a characteristic curve exhibits a gentle slope and, in the ordinary state, the operating mode is shifted from the mountain climbing mode to the closed loop mode earlier than the timing when the operating mode enters the closed loop mode and the driving speed is reduced. Consequently, there occurs a problem such that it takes a time until the object image reaches the in-focused point. On the contrary, when the focal distance is large and the angle of view is small, the depth of field contrarily becomes shallow, so that the shape of mountain exhibits a sharp slope and it becomes difficult to enter the closed loop mode just before the in-focused point. Thus, there are problems such that even when the driving speed is reduced, the object image passes the in-focused point due to a backlash of the lens driving system or the like and an overshoot occurs. In recent years, in the video camera having a zoom lens as a standard equipment, those problems are unescapable.